PROJECT SUMMARY Adolescents, particularly those growing up in urban communities with high rates of poverty and crime, experience high levels of exposure to violence as both witnesses and victims. Exposure to violence during adolescence has been associated with a host of physical and mental health problems with serious consequences for individuals and society. Prior research has established that youth frequently exposed to violence are more likely to engage in aggressive behavior, but the causal relation between violence exposure and aggression, and the underlying factors that account for this relation have not been clearly established. The goal of this project is to clarify the relation between violence exposure (both witnessing and victimization) and aggressive behavior during adolescence, and to identify mechanisms that account for this relation. This project differs from prior studies that have examined between-person factors that influence trajectories of violent behavior and violence exposure across the life course. Its focus is on time-specific within-person factors (i.e., mediators) within each grade of middle school that influence within-individual (i.e., within-person) changes during early adolescence. Early adolescence is a particularly salient time for such a focus because of the many changes that increase adolescents? risk of exposure to violence, victimization, and aggression. Furthermore, the most common trajectory of antisocial behavior has its onset during adolescence and is related to social processes and peer influences that emerge during this time. Establishing the underlying developmental process is critical both for understanding the etiology of aggressive behavior and for identifying optimal points of intervention to deflect trajectories of problem behavior. This project addresses these goals through secondary analysis of a rich data set from a project that collected data on measures of witnessing violence, victimization, physical aggression, beliefs and values, and delinquent peer associations every 3 months through all 3 years of middle school. Participants were 1,795 adolescents from seven cohorts at three public middle schools in urban communities with high rates of crime and violence. They ranged from 10 to 17 years old (83% African American; 47% male). Each cohort was assessed four times per year for up to 3 years with collateral data from teachers? ratings. Examining changes across these 12 waves will allow us to examine changes both within and across all three years of middle school and during the summers between school years. This project?s specific aims are to: 1) Investigate reciprocal relations between violence exposure and aggression during each grade of middle school. 2) Determine the extent to which beliefs and values mediate the impact of adolescents? exposure to violence on aggressive behavior. 3) Determine the extent to which peer associations mediate the impact of aggression on exposure to violence. These aims will be addressed using recently developed approaches to latent curve modeling that provide more direct and refined tests of hypotheses about both between-person and within-person changes than has previously been possible.